Turbines blowin' in the wind 100 miles offshore

Portland, Ore. -- "You don't need a weather man to know which way the wind blows," Bob Dylan sang in "Subterranean Homesick Blues." That sentiment rings especially true if your turbine is blowin' in the wind more than 100 miles offshore, because there the breeze is always stiff. Now a designer of offshore drilling rigs for oil companies, MIT engineering professor Paul Sclavounos, has validated the blueprints for an extra-large, 5-megawatt floating wind turbine with the National Renewable Energy Laboratory.

"After two years of performing simulations on the wave and wind loading of this structure and the motion responses in severe weather, we have very favorable re- sults," said Sclavounos. "We are now ready to build a slightly scaled-down prototype where we can measure the actual behavior of the structure, and hopefully enlist a commercial industrial sponsor to start building offshore wind farms."

Wind farms began on land, as eyesores familiar to anyone who drives much (for instance, from Los Angeles to Palm Springs). But the lack of constant wind onshore and the steady stream of complaints from the public have prompted a migration offshore. The public continues to complain about wind farms when they are visible from shore, prompting the newer projects to move even farther out. Last month, a consortium including oil company Talisman Energy Inc. and Scottish and Southern Energy began testing a 5-MW wind turbine almost 10 miles off the coast of Moray Firth, Scotland, where at 150 feet it is shallow enough to utilize an underwater foundation.

For the United States, Sclavounos proposes abandoning traditional rigid attachments to the ocean bottom, freeing the floating wind turbines to be located on the high seas, where the wind blows hardest. The Scottish project, if successful, will be a farm of 200 turbines. Sclavounos plans to up the ante in the United States by designing a farm twice as big--400 turbines, to power about 100,000 homes--located 100 miles off the New England coast.

"The ocean gets deeper much faster off the shore of the Northeastern United States coast than in Scottish waters or the North Sea," said Stephen Connors, director of the Analysis Group for Regional Energy Alternatives at MIT's Lab for Energy and the Environment. Connors is evaluating how economical offshore wind turbines could be. "The wind blows from half again up to twice as hard off the Northeast coast than it does onshore, so our turbines will produce correspondingly more power, making them economical even if they cost twice as much to install so far out."

The floating platform itself will cost only about one-third of what the deep-water foundations cost for the Scottish project, Connors said. However, the electrical system will cost much more because of the 100 miles of undersea cable that will need to be laid. The final cost and energy efficiency of the project will be evaluated after the first prototype is installed.

MIT's design, called a tension leg platform, uses long steel cables to tether the corners of the platform to giant concrete-block anchors dropped down to the ocean floor. The platform, which measures 100 feet in diameter, then floats atop water as deep as 650 feet. The tower stands about 300 feet above the water with three rotors that are 450 feet in diameter and an underwater concrete ballast. These massive structures would be completely assembled onshore, and then towed out and anchored by a tugboat.

"According to our simulations, even in the worst hurricane winds our platforms would only move about 2 meters," said Sclavounos. "And even the highest waves would remain well below the blades."

Now that the design has proven itself in simulation by the Ship Wave Analysis Software Suite, Sclavounos plans to build a real prototype onshore, then tow it out several miles off the coast. "Next, we hope to install a half-scale prototype south of Cape Cod so that we can show it really will float and behave as well as our simulations predict," said Sclavounos. "Then we will be in a position to make a very good cost analysis to see if the economics will be as favorable as we think they will be."